The present invention relates to what is called a "joystick".
One of conventional joysticks is of what is called a "torsion spring type" in which a torsion spring is engaged with a rotary shaft of each of a pair of variable resistors so as to automatically return a control stick to its central neutral position, as described in Japanese Utility Model Application Unexamined Publication No. Sho. 55-17379. However, in such a joystick, since a force for holding the control stick is determined by the resilient forces of the torsion springs, the holding force varies depending on the tilt angle and direction of the control stick as shown in FIG. 3. In FIG. 3, the symbols accompanying the coordinates, .theta..sub.1, .theta..sub.2 and F, represent the tilt angles in the X and Y directions and the stick-holding force, respectively. The variation of the holding force greatly deteriorates the ease of manipulating the control stick. Further, the holding force, which is a force for returning the control stick back to the central position, becomes smaller, as the tilt angle decreases. This in turn reduces the accuracy in returning the control stick to the central position.
To solve the above problems, the present applicant developed a joystick of a "coil spring type" in which a control stick is fitted with a single coil spring at its lower portion so that it can be elongated and compressed, and the lower end part of the control stick assembly slides, in pressure contact, on the inside surface of a guide case having a generally V-shaped cross-section, as disclosed in Japanese Utility Model Application Unexamined Publication No. Sho. 63-188906.
In the joystick of this type, the control stick is automatically returned to its central position by the resilient force of the coil spring fitted to the control stick in its lower portion. Since the control stick holding force is determined by the single coil spring, the holding force becomes uniform for any tilt direction of the control stick in a concentric circle on the inside surface of the guide case. However, even in the joystick of this type, which is simply equipped with the guide case having the cup-shaped inside surface, the holding force still changes depending on the tilt angle of the stick, resulting in a subtle variation in the manipulation feeling of the stick.